1. Field of the Invention
The present invention relates to a semiconductor apparatus using an amorphous oxide semiconductor, and to a method of manufacturing the same. In particular, the present invention relates to a semiconductor apparatus including a plurality of amorphous oxide semiconductors having different compositions, and to a method of manufacturing the same. In addition, the present invention relates to a semiconductor apparatus including a plurality of thin film transistors having different threshold voltages, and to a method of manufacturing the same.
2. Description of the Related Art
A field effect transistor (FET) is a 3-terminal device having a gate electrode, a source electrode, and a drain electrode. The FET is an electronic active element having a function of switching the current flowing between the source electrode and the drain electrode by applying a voltage to the gate electrode to control a current flowing through a channel layer.
Particularly, an FET formed by using a thin film as a channel layer, which is formed on an insulating substrate made of, for example, ceramics, glass, or plastic, is called a thin film FET (thin film transistor, TFT).
The TFT is formed by using a thin film technology, so that the TFT has an advantage of being easily formed on the substrate having a relatively large area, and therefore is widely used as a drive element for a flat panel display element such as a liquid crystal display element.
In other words, an active liquid crystal display element (ALCD) turns on/off each image pixel by using TFTs formed on a glass substrate.
Further, in a future high-performance organic LED display (OLED), current drive for each pixel by TFTs is thought to be effective.
In addition, a liquid crystal display device having a higher performance is realized in which a TFT circuit having a function of driving and controlling an entire image is formed on a substrate placed in the periphery of an image display region.
A TFT which is most widely used at present is an element having a channel layer formed of a polysilicon film or an amorphous silicon film. The TFT is called a metal-insulator-semiconductor field effect transistor (MIS-FET) device.
For driving pixels, an amorphous silicon TFT is put into practical use, and for driving and controlling the entire image, a polysilicon TFT having a high performance is put into practical use.
However, it is difficult to form the amorphous silicon TFT and the polysilicon TFT on the substrate such as a plastic plate or a film because a high-temperature process is necessary for forming a device.
On the other hand, in recent years, development for realizing a flexible display by forming TFTs on a polymer plate or a film and using the TFTs as drive circuits for an LCD or an OLED has been vigorously conducted.
As a material capable of being formed on the plastic film or the like, organic semiconductor films which can be formed at low temperature and exhibits electric conductivity have been attracting attention.
For example, as the organic semiconductor films, research and development of pentacene or the like are advanced.
These organic semiconductor films each include an aromatic ring, and a large carrier mobility is obtained in a stacking direction of the aromatic rings when they are crystallized.
For example, it is reported that, in a case where pentacene is used as a channel layer, the carrier mobility is about 0.5 cm2(Vs)−1 which is equal to that of an amorphous Si-MOSFET.
However, the organic semiconductor such as pentacene is disadvantageous in that thermal stability thereof is low (<150° C.), so that a practical device is not realized.
Recently, as a material applicable to the channel layer of TFT, an oxide material has been attracting attention.
For example, development of the TFT in which a transparent conductive oxide polycrystalline thin film mainly made of ZnO is used as the channel layer has been vigorously conducted.
The above-mentioned thin film can be formed at relatively low temperature and can be formed on the substrate such as a plastic plate or a plastic film.
However, a compound mainly made of ZnO cannot form a stable amorphous phase at room temperature, whereby it becomes a polycrystalline phase. Accordingly, it is impossible to increase an electron mobility by scattering in a polycrystalline grain boundary.
In addition, each shape of the polycrystalline particles and interconnection therebetween largely vary depending on a film forming method, so that characteristics of the TFT device also vary.
K. Nomura et al., Nature 432, 488 (2004) discloses a thin film transistor using an In—Ga—Zn—O-based amorphous oxide.
The transistor can be formed on a plastic substrate or a glass substrate at room temperature.
Further, transistor characteristics of a normally-off type transistor are obtained when a field effect mobility is about 6 to 9. In addition, the transistor has a feature of being transparent with respect to visible light.
On the other hand, if it is possible to produce a plurality of thin film transistors having different threshold voltages on a substrate, a high-performance circuit or a circuit suitable for an application uses can be achieved.
For example, Japanese Patent Application Laid-Open No. 2004-128487 discloses a technology of producing thin film transistors having different threshold voltages in a silicon-based TFT, and describes that the technology enables improvement of a circuit performance, for example, reduction in power consumption.
In the silicon-based thin film transistor, an element such as boron is doped in a channel layer so as to control the threshold voltage.
In order to produce TFTs having different threshold voltages, a doping concentration is adjusted for each of the TFTs having different threshold voltages.
A doping concentration is as small as about 1015 to 1018 atoms/cm3, so that a method of controlling the doping concentration is limited, and ion implantation is generally employed.
According to the method, in order to form regions having different doping concentration on the substrate, it is necessary to adjust an amount of implanted ions for each region, and therefore repeat a step of implanting ions a plurality of times, thereby complicating the step.
On the other hand, a method of controlling the threshold voltage of the thin film transistor in which the amorphous oxide film is applied to the channel layer is unknown.